1. Field of the Invention
This invention relates to a reciprocating screw-type injection molding machine, more particularly to a reciprocating screw for an injection molding machine.
2. Description of the Related Art
Referring to FIG. 1, a conventional reciprocating screw-type injection molding machine is shown to include a machine bed 1, a mold mechanism 2 and an injecting mechanism 3. The injecting mechanism 3 includes a heating extruder barrel 30 with an extruder head 31, a reciprocating screw 32, a check valve 33 and a hopper 34. The screw 32 is mounted coaxially and rotatably in the barrel 30. The hopper 34 is coupled with a tail portion 35 of the barrel 30 for feeding plastic granules into the barrel 30. Upon rotation of the screw 32 in the barrel 30, the plastic granules move from the tail portion 35 to the extruder head 31. Ideally, during the movement of the plastic granules in the barrel 30, an inner surface of the barrel 30 rubs and heats the plastic granules to melt entirely the same as the latter arrives in the extruder head 31 for injection into the mold mechanism 2. However, because it is difficult for the inner surface of the barrel 30 to contact the plastic granules which are located on and near the screw 32, when arriving in the extruder head 31, some of the plastic granules have not yet been molten entirely, thereby affecting adversely the quality of the products molded using the conventional injection molding machine. For example, the molded products cannot have a uniform color or uniform stress. Additionally, undesired air spaces are formed in the molded products.
To overcome this problem, different kinds of screws have been developed. As shown in FIG. 2, a first kind of improved screw 320 has several enlarged shank sections, which are formed with inclined grooves (A). As shown in FIG. 3, a second kind of improved screw 321 has an enlarged shank section, which is formed with a plurality of axially extending slots (B). As shown in FIG. 4, a third kind of improved screw 322 has a non-uniform pitch and a non-uniform thread angle. Although the improved structures of these screws 320, 321 and 322 can increase the uniformity in the interior structure of the molded products, it is difficult to manufacture the same due to their complex constructions. In addition, the complex structures of the screws 320, 321 and 322 can cause difficulties in estimating the heat caused by friction between the outer surfaces of the screws 320, 321 and 322.